Is it possible to increase the dynamic range of 'External Quick Edit'?
I did come with a procedure which uses EXR files

Goal

To be able to create >8 bit color depth when creating texture camera projections

Procedure

Blender v2.7.6.2, GIMP v2.9.3

I created an example file to illustrate the workflow


Make 32 bit texture (UV-Tex) for the UV texture
Set Cycles as your renderer
Set Output for render to OpenEXR Float(Half), RGBA
Set lights, cameras
Create material for your model
Create a texture node for baking, set it to UV-Tex, leave active (with orange border)
Bake
Save result of baking to EXR file
Clean UV-Tex for projection
Select a camera, render, save result as EXR Float(half) RENDER
Edit EXR to suit your needs
Switch to 'Texture Paint Mode' Display object as 'Texture'
You should have 'External' tab now in the toolbox in 3D view.
In the Node view go to the texture node, and upload EXR RENDER file
(This step is important, if you don't make it active in node editor, and try to project EXR image you will get "RuntimeError: Error: Image data could not be found")
Switch Texture node back to UV-Tex
In "External" menu click "Apply Camera Image" select RENDER file
UV-Tex should have your projected EXR image now


All is working fine, however the very last step degrades the image to 8-bit depth, even if the final file is 32 bit. I am trying to figure out if it can be fixed, and will update this post, if I'll succeed.

ADDENDUM The Issue had been solved in Blender master branch and high definition projections should be possible from that moment on, Thanks Campbell!